Dynamics of soil biomass C,N, and P in a dry tropical forest in India

Summary Three dry tropical forest soils along a topographic sequence were examined to determine the seasonal dynamics of microbial C, N, and P. The lowest microbial biomass was found in forest soils at the foot of the hill followed by midslope forest soils. The hilltop soil, which had the most fine particles, water-holding capacity, organic C, and total N, reflected the presence of greater amounts of microbial C, N, and P. Mean annual microbial C, N, and P ranges were 466–662, 48–72 to 21–30 g g^-1, respectively. The seasonal pattern of microbial biomass, C, N, and P was similar at all sites, the values being greatest during the dry season and lowest during the wet season. The seasonal values for microbial biomass C, N, and P were positively correlated with each other and a negative correlation was found between microbial biomass and the fine root mass in these forest soils.     

Effects of farmyard manure and inorganic fertilizer on the dynamics of soil microbial biomass in a tropical dryland agroecosystem

Changes in the soil microbial biomass following applications of farmyard manure and inorganic fertilizer, alone and in combination, were studied for two annual cycles in a rice-lentil crop sequence grown under rainfed tropical dryland conditions. During the two annual cycles the microbial biomass C range (g g^-1) was 146–241 (x = 204), 191–301 (245), 244–382 (305), and 294–440 (365) in control, fertilizer, manure and manure+fertilizer plots, respectively. The corresponding ranges for microbial biomass N (g g^-1) were 16.5–21.0 (19.5), 20.4–38.2 (26.0), 23.0–34.6 (27.0) and 26.2–42.4 (33.3), and for microbial biomass P (g g^-1) 4.4–8.2 (7.0) 6.0–11.2 (9.6), 11.2–22.0 (17.0), and 10.0–25.4 (18.3). The maximum increase in the microbial biomass, due to these inputs was observed under the manure+fertilizer treatment followed, in decreasing order, by manure alone and fertilizer alone. Within individual crop periods the levels of microbial biomass decreased sharply from the seedling to the flowering stage and then increased slightly with crop maturity. The maximum levels of microbial biomass C and P were observed during the summer fallow. The maximum accumulation of microbial biomass N occurred in the early rainy season, immediately after the soil amendments. Microbial biomass C, N, and P were positively related to each other throughout the annual cycle.     

Microbial biomass turnover and enzyme activities following the application of farmyard manure to field soils with and without previous long-term applications

Summary We studied the build-up and turnover of microbial biomass following the addition of farmyard manure to an unmanured soil and to soils from a long-term experiment in which different levels of farmyard manure had been applied for the last 23 years. The application of farmyard manure at 15–90 t ha^-1 to previously unmanured soil increased the microbial biomass during the first 3 months of incubation but a gradual decline occurred with further incubation for up to 12 months. Microbial biomass C was positively correlated with soil organic C and ranged from 1.8% to 2.2% of organic C after 12 months of farmyard manure applications. Biomass turnover increased with the application of farmyard manure, ranging from 0.81 to 0.87 year^-1 with various levels of manure. Amendment of soils from the long-term manure experiment with various levels of farmyard manure led to a build-up and decline in biomass C as seen in the unmanured soils, but biomass C was higher in all treatments compared to the corresponding unmanured soil treatments. Biomass turnover was greater compared to the unmanured soil treatments and it decreased with increasing levels of farmyard manure. The average soil respiratory activity increased with increasing levels of farmyard manure, but respiratory activity per unit of biomass C decreased with increasing levels of manure. Enzyme activities were greater in long-term manured soils compared to unmanured soils amended with various levels of manure. There was a significant correlation between biomass C and enzyme activities.     

Effect of residue placement and chemical fertilizer on soil microbial biomass under tropical dryland cultivation

Four treatments (control, chemical fertilizer, wheat straw, and wheat straw+fertilizer) were established on the dryland experimental farm of the Institute of Agricultural Sciences, Banaras Hindu University. Organic in C in the different treatments ranged from 0.69 to 0.93%, total N from 0.08 to 0.11%, and total P from 0.018 to 0.021. The application of straw significantly increased the soil water-holding capacity. The maximum effect on the microbial biomass was realized with the straw+fertilizer treatment, followed by straw and then by the fertilizer treatment. During the study microbial biomass C ranged from 144 to 491 g g^-1 dry soil, biomass N from 14.6 to 50.1 g g^-1, and biomass P from 7.2 to 17.6 g g^-1 soil. Microbial biomass C, N and P represented 3.2–4.6% of total C, 2.6–3.8% of total N, and 5.8–8.2% of total P in the soil, respectively, in all cases the highest proportion occurred in the straw+fertilizer treatment and the lowest in the control. Microbial biomass C, N, and P were positively correlated with each other. Microbial biomass C and N increased by 77% in straw+fertilizer-treated plots relative to the control. The increase in microbial biomass P in the straw+fertilizer treatment over the control was 81%. The increase in the microbial biomass is expected to enhance nutrient availability in the soil, as the microbial biomass acts both as a sink and a source of plant nutrients.     

Activity and biomass of soil microorganisms at different depths

We measured microbial biomass C and soil organic C in soils from one grassland and two arable sites at depths of between 0 and 90 cm. The microbial biomass C content decreased from a maximum of 1147 (0–10 cm layer) to 24 g g^-1 soil (70–90 cm layer) at the grassland site, from 178 (acidic site) and 264 g g^-1 soil (neutral site) at 10–20 cm to values of between 13 and 12 g g^-1 soil (70–90 cm layer) at the two arable sites. No significant depth gradient was observed within the plough layer (0–30 cm depth) for biomass C and soil organic C contents. In general, the microbial biomass C to soil organic C ratio decreased with depth from a maximum of between 1.4 and 2.6% to a minimum of between 0.5 and 0.7% at 70–90 cm in the three soils. Over a 24-week incubation period at 25°C, we examined the survival of microbial biomass in our three soils at depths of between 0 and 90 cm without external substrate. At the end of the incubation experiment, the contents of microbial biomass C at 0–30 cm were significantly lower than the initial values. At depths of between 30 and 90 cm, the microbial biomass C content showed no significant decline in any of the four soils and remained constant up to the end of the experiment. On average, 5.8% of soil organic C was mineralized at 0–30 cm in the three soils and 4.8% at 30–90 cm. Generally, the metabolic quotient qCO₂ values increased with depth and were especially large at 70–90 cm in depth.     

The influence of tillage on the proportion of organic carbon and nitrogen in the microbial biomass of medium-textured soils in a humid climate

Summary Microbial biomass C and N respond rapidly to changes in tillage and soil management. The ratio of biomass C to total organic C and the ratio of mineral N flush to total N were determined in the surface layer (0–5 cm) of low-clay (8–10%), fine sandy loam, Podzolic soils subjected to a range of reduced tillage (direct drilling, chisel ploughing, shallow tillage) experiments of 3–5 years' duration. Organic matter dynamics in the tillage experiments were compared to long-term conditions in several grassland sites established on the same soil type for 10–40 years. Microbial biomass C levels in the grassland soils, reduced tillage, and mouldboard ploughing treatments were 561, 250, and 155 g g^-1 soil, respectively. In all the systems, microbial biomass C was related to organic C (r=0.86), while the mineral N flush was related to total N (r=0.84). The average proportion of organic C in the biomass of the reduced tillage soils (1.2) was higher than in the ploughed soils (0.8) but similar to that in the grassland soils (1.3). Reduced tillage increased the average ratio of mineral N flush to total soil N to 1.9, compared to 1.3 in the ploughed soils. The same ratio was 1.8 in the grassland soils. Regression analysis of microbial biomass C and percent organic C in the microbial biomass showed a steeper slope for the tillage soils than the grassland sites, indicating that reduced tillage increased the microbial biomass level per unit soil organic C. The proportion of organic matter in the microbial biomass suggests a shift in organic matter equilibrium in the reduced tillage soils towards a rapid, tillage-induced, accumulation of organic matter in the surface layer.     

The spatial distribution of soil microbial biomass in a permanent row crop

The effects of soil texture (silt loam or sandy loam) and cultivation practice (green manure) on the size and spatial distribution of the microbial biomass and its metabolic quotient were investigated in soils planted with a permanent row crop of hops (Humulus lupulus). The soil both between and in the plant rows was sampled at three different depths (0–10, 10–20, and 20–30 cm). The silt loam had a higher overall microbial biomass C concentration (260 g g^-1) than the sandy loam (185 g g^-1), whereas the sandy loam had a higher (3.1 g CO₂-C mg^-1 microbial Ch^-1) metabolic quotient than the silt loam (2.6 g CO₂-C mg^-1 microbial C h^-1), on average over depth (0–30 cm) and over all treatments. There was a sharp decrease in the microbial biomass with increasing depth for all plots. However, this was more pronounced in the silt loam than in the sandy loam. There was no distinct influence of sampling depth on the metabolic quotient. The microbial biomass was considerably higher in the rows than between the rows, especially in the silt loam plots. There was no significant difference between plots without green manure and plots with green manure for either the microbial biomass or the metabolic quotient.     

Influence of soil properties on microbial C,N, and P in dry tropical ecosystems

Summary Relationships between soil physicochemical characteristics and soil microbial C, N, and P in Indian dry tropical ecosystems are discussed. The major ecosystem studies were on forest, savanna, cropped fields, and mine spoils. The highest microbial C, N, and P levels were recorded from the mixed forest and the lowest levels in 5-year-old mine spoil. Across the sites, microbial C ranged from 226 to 643 g g^-1, microbial N from 19 to 71 g g^-1, and microbial P from 9 to 28 g g^-1 soil. The proportion of soil organic C contained in the microbial biomass ranged from 2.2 to 5.0%. The microbial C: N ratio in these soils ranged from 7.4. to 13.1 and the microbial C: P ratio from 16.6 to 30.6. The concentrations of microbial C, N, and P were correlated with several soil properties and among themselves. The soil properties, in various linear combinations, explained 90–99% of the variability in the microbial nutrients. Grazing of the savanna had some effect on the level of microbial biomass, and as the mine spoil aged, the level of microbial C, N, and P also increased.     

Natural nitrogen-15 abundance and carbohydrate content and composition of organic matter particle-size fractions from a sandy soil

Sandy soil samples collected from under a woody/grass savanna in the Lamto experimental area (6°13N, 5°20W; Côte dIvoire, West Africa), were fractionated according to particle size with the aim of measuring the natural abundance of ¹⁵N and determining the contents and composition of hydrolysable carbohydrates of soil organo-mineral particles for a better understanding of the contribution of each individual fraction to the soil function. The contributions of the fractions <20 m to the total pool of organic matter were 77% for C and 84% for N. Larger amounts of carbohydrates were found in the clay and silt fractions (3,784–6,043 g g^–1 soil). The carbohydrate composition indicated that microbe-derived carbohydrates [e.g. galactose (Gal) and mannose (Man)] accumulated preferentially in the fine fractions while plant-derived sugars [e.g. arabinose (Ara) and xylose (Xyl)] were dominant in coarse fractions. A negative relationship was observed between C:N ratio and ¹⁵N natural abundance on the one hand, and on the other hand between C:N and (Gal+Man):(Ara+Xyl), Man:(Ara+Xyl) and Man:Xyl ratios, clearly indicating that the chemistry of the organic materials of the particle-size fractions reflects a change from soil chemistry dominated by plant materials to that dominated by microbial biomass and metabolites. The contribution of a given fraction to soil microbial activity is controlled by the quality or quantity of associated soil organic matter, its microbial biomass and also by the accumulation of microbial-derived carbohydrates which can be resynthesized or recycled.     

Substrate type,temperature, and moisture content affect gross and net N mineralization and nitrification rates in agroforestry systems

Accurate prediction of soil N availability requires a sound understanding of the effects of environmental conditions and management practices on the microbial activities involved in N mineralization. We determined the effects of soil temperature and moisture content and substrate type and quality (resulting from long-term pasture management) on soluble organic C content, microbial biomass C and N contents, and the gross and net rates of soil N mineralization and nitrification. Soil samples were collected at 0–10 cm from two radiata pine (Pinus radiata D. Don) silvopastoral treatments (with an understorey pasture of lucerne, Medicago sativa L., or ryegrass, Lolium perenne L.) and bare ground (control) in an agroforestry field experiment and were incubated under three moisture contents (100, 75, 50% field capacity) and three temperatures (5, 25, 40 °C) in the laboratory. The amount of soluble organic C released at 40 °C was 2.6- and 2.7-fold higher than the amounts released at 25 °C and 5 °C, respectively, indicating an enhanced substrate decomposition rate at elevated temperature. Microbial biomass C:N ratios varied from 4.6 to 13.0 and generally increased with decreasing water content. Gross N mineralization rates were significantly higher at 40 °C (12.9 g) than at 25 °C (3.9 g) and 5 °C (1.5 g g^–1 soil day^–1); and net N mineralization rates were also higher at 40 °C than at 25 °C and 5 °C. The former was 7.5-, 34-, and 29-fold higher than the latter at the corresponding temperature treatments. Gross nitrification rates among the temperature treatments were in the order 25 °C >40 °C >5 °C, whilst net nitrification rates were little affected by temperature. Temperature and substrate type appeared to be the most critical factors affecting the gross rates of N mineralization and nitrification, soluble organic C, and microbial biomass C and N contents. Soils from the lucerne and ryegrass plots mostly had significantly higher gross and net mineralization and nitrification rates, soluble organic C, and microbial biomass C and N contents than those from the bare ground, because of the higher soil C and N status in the pasture soils. Strong positive correlations were obtained between gross and net rates of N mineralization, between soluble organic C content and the net and gross N mineralization rates, and between microbial biomass N and C contents.     

Influence of inorganic fertilizers and organic amendments on soil organic matter and soil microbial properties under tropical conditions

 Soil organic matter level, mineralizable C and N, microbial biomass C and dehydrogenase, urease and alkaline phosphatase activities were studied in soils from a field experiment under a pearl millet-wheat cropping sequence receiving inorganic fertilizers and a combination of inorganic fertilizers and organic amendments for the last 11 years. The amounts of soil organic matter and mineralizable C and N increased with the application of inorganic fertilizers. However, there were greater increases of these parameters when farmyard manure, wheat straw or Sesbania bispinosa green manure was applied along with inorganic fertilizers. Microbial biomass C increased from 147 mg kg^–1 soil in unfertilized soil to 423 mg kg^–1 soil in soil amended with wheat straw and inorganic fertilizers. The urease and alkaline phosphatase activities of soils increased significantly with a combination of inorganic fertilizers and organic amendments. The results indicate that soil organic matter level and soil microbial activities, vital for the nutrient turnover and long-term productivity of the soil, are enhanced by use of organic amendments along with inorganic fertilizers. Received: 6 May 1998     

Mineralization of carbon and nitrogen from fresh and anaerobically stored sheep manure in soils of different texture

A sandy loam soil was mixed with three different amounts of quartz sand and incubated with (¹⁵NH₄)₂SO₄ (60 g N g^-1 soil) and fresh or anaerobically stored sheep manure (60 g g^-1 soil). The mineralization-immobilization of N and the mineralization of C were studied during 84 days of incubation at 20°C. After 7 days, the amount of unlabelled inorganic N in the manure-treated soils was 6–10 g N g^-1 soil higher than in soils amended with only (¹⁵NH₄)₂SO₄. However, due to immobilization of labelled inorganic N, the resulting net mineralization of N from manure was insignificant or slightly negative in the three soil-sand mixtures (100% soil+0% quartz sand; 50% soil+50% quartz sand; 25% soil+75% quartz sand). After 84 days, the cumulative CO₂ evolution and the net mineralization of N from the fresh manure were highest in the soil-sand mixutre with the lowest clay content (4% clay); 28% fo the manure C and 18% of the manure N were net mineralized. There was no significant difference between the soil-sand mixtures containing 8% and 16% clay, in which 24% of the manure C and -1% to 4% of the manure N were net mineralized. The higher net mineralization of N in the soil-sand mixture with the lowest clay content was probably caused by a higher remineralization of immobilized N in this soil-sand mixture. Anaerobic storage of the manure reduced the CO₂ evolution rates from the manure C in the three soil-sand mixtures during the initial weeks of decomposition. However, there was no effect of storage on net mineralization of N at the end of the incubation period. Hence, there was no apparent relationship between net mineralization of manure N and C.     

Carbon and nitrogen relationships in the microbial biomass of soils in beech (Fagus sylvatica L.) forests

Soils from 38 German forest sites, dominated by beech trees (Fagus sylvatica L.) were sampled to a depth of about 10 cm after careful removal of overlying organic layers. Microbial biomass N and C were measured by fumigation-extraction. The pH of the soils varied between 3.5 and 8.3, covering a wide range of cation exchange capacity, organic C, total N, and soil C:N values. Maximum biomass C and biomass N contents were 2116 g C m^-2 and 347 g N m^-2, while minimum contents were 317 and 30 g m^-2, respectively. Microbial biomass N and C were closely correlated. Large variations in microbial biomass C:N ratios were observed (between 5.4 and 17.3, mean 7.7), indicating that no simple relationship exists between these two parameters. The frequency distribution of the parameters for C and N availability to the microflora divided the soils into two subgroups (with the exception of one soil): (1) microbial: organic C>12 mg g^-1, microbial:total N>28 mg g^-1 (n=23), a group with high C and N availability, and (2) microbial:organic C12 mg g^-1, microbial:total N28 mg g^-1 (n=14), a group with low C and N availability. With the exception of a periodically waterlogged soil, the pH of all soils belonging to subgroup 2 was below 5.0 and the soil C:N ratios were comparatively high. Within these two subgroups no significant correlation between the microbial C:N ratio and soil pH or any other parameter measured was found. The data suggest that above a certain threshold (pH 5.0) microbial C:N values vary within a very small range over a wide range of pH values. Below this threshold, in contrast, the range of microbial C:N values becomes very large.     

Effects of animal manure and mineral fertilizer on the total carbon and nitrogen contents of soil size fractions

Summary Soil was sampled in autumn 1984 in the 132 field (sandy loam soil) of the Askov long-term experiments (started in 1894) and fractionated according to particle size using ultrasonic dispersion and sedimentation in water. The unmanured plot and plots given equivalent amounts of N (1923–1984 annual average, 121 kg N/ha) in either animal manure or mineral fertilizer were sampled to a depth of 15 cm, fractionated and analysed for C and N. Mineral fertilizer and animal manure increased the C and N content of whole soil, clay (<2 m) and silt (2–20 m) size fractions relative to unmanured samples, while the C content of the sand size fractions (fine sand 1, 20–63 m; fine sand 2, 63–200 m; coarse sand, 200–2000 m) was less affected. Clay contained 58% and 65°70 of the soil C and N, respectively. Corresponding values for silt were 30% and 26%, while sand accounted for 10% of the soil C. Fertilization did not influence this distribution pattern. The C : N ratio of the silt organic matter (14.3) was higher and that of clay (10.6) lower than whole-soil C:N ratios (12.0). Fertilization did not influence clay and silt C : N ratios. Animal manure caused similar relative increases in the organic matter content of clay and silt size fractions (36%). In contrast, mineral fertilizer only increased the organic matter content of silt by 21% and that of clay by 14%.     

Synthesis of 15N-labelled microbial biomass in soil in situ and extraction of biomass N

Summary A Pakistani soil (Hafizabad silt loam) was incubated at 30°C with varying levels of ¹⁵N-labelled ammonium sulphate and glucose (C/N ratio of 30 at each addition rate) in order to generate different insitu levels of ¹⁵N-labelled microbial biomass. At a stage when all of the applied ¹⁵N was in organic forms, as biomass and products, the soil samples were analysed for biomass N by the chloroform (CHCl₃) fumigation-extraction method, which involves exposure of the soil to CHCl₃ vapour for 24 h followed by extraction with 500 mM K₂SO₄. A correction is made for inorganic and organic N in 500 mM K₂SO₄ extracts of the unfumigated soil. Results obtained using this approach were compared with the amounts of immobilized ¹⁵N extracted by 500 mM K₂SO₄ containing different amounts of CHCl₃. The extraction time varied from 0.5 to 4 h.The amount of N extracted ranged from 27 to 270 g g^–1, the minimum occurring at the lowest (67 g g^–1) and the maximum at the highest (333 g g^–1) N-addition rate. Extractability of biomass ¹⁵N ranged from 25% at the lowest N-addition rate to 65%a for the highest rate and increased consistently with an increase in the amount of ¹⁵N and glucose added. The amounts of both soil N and immobilized ¹⁵N extracted with 500 mM K₂SO₄ containing CHCl₃ increased with an increase in extraction time and in concentration of CHCl₃. The chloroform fumigation-extraction method gives low estimates for biomass N because some of the organic N in K₂SO₄ extracts of unfumigated soil is derived from biomass.     

Carbon and nitrogen dynamics in a phosphorus-deficient soil amended with organic residues and fertilizers in western Kenya

The contribution of organic resources to the restoration of soil fertility in smallholder farming systems in East Africa is being tested as an alternative to costly fertilizers. Organic inputs are expected to have advantages over fertilizers by affecting many biochemical properties controlling nutrient cycling. Our study examined changes in soil C and N, C and N mineralization, microbial biomass C (MBC) and N (MBN), and particulate organic matter (POM) in a P-limiting soil in western Kenya after applications of organic residues and fertilizers to overcome P limitation to crops. Leaf biomass from six different tree (shrub) species was incorporated into the soil at 5 Mg ha^–1 for five consecutive maize growing seasons, over 2.5 years. Triple superphosphate was applied separately at 0, 10, 25, 50, and 150 kg P ha^–1 in combination with 120 kg N ha^–1 as urea. Soil inorganic N, soil organic C, mineralizable N, and total C in all POM fractions and total N in the 53- to 250-m POM fraction increased following addition of all organic residues compared to the control. Whether there was an advantage of organic residue incorporation over inorganic fertilizer use depended on the soil parameter studied, the organic residue and the rate of fertilization. Most differences were found in N mineralization where 14.4–21.6 mg N kg^–1 was mineralized in fertilizer treatments compared to 25.2–30.5 mg N kg^–1 in organic residue treatments. C and N mineralization and the 53- to 250-m POM fractions were the most sensitive parameters, correlating with most of the studied parameters. Organic residues can contribute to improved soil nutrient cycling while the magnitude of their contribution depends on the biochemical properties of the residues.     

The influence of mineral and organic fertilisers on the growth of the endogeic earthworm Octolasion tyrtaeum (Savigny)

Endogeic earthworms play an important role in mobilisation and stabilisation of carbon and nitrogen in forest and arable soils. Soil organic matter is the major food resource for endogeic earthworms, but little is known about the size and origin of the organic matter pool on which the earthworms actually live. We measured changes in body mass of juvenile endogeic earthworms, Octolasion tyrtaeum (Savigny), in soils with different C and N contents resulting from different fertiliser treatments. The soil was taken from a long-term experiment (Statischer Düngungsversuch, Bad Lauchstädt, Germany). The treatments included (1) non-fertilised soil, (2) NPK fertilised soil, (3) farmyard manure fertilised soil and (4) NPK + farmyard manure fertilised soil. The soil was incubated in microcosms with and without one juvenile O. tyrtaeum for 80 days.Earthworm biomass decreased in non-fertilised soil by 48.6%, in NPK soil by 9.4%, but increased in farmyard manure soil by 19.7% and 42.8% (soil with additional NPK application). In farmyard manure treatments the biomass of bigger individuals decreased, but in smaller individuals it increased. In NPK fertilised soil without farmyard manure only small O. tyrtaeum increased in body mass, whereas in the non-fertilised soil all individuals decreased in body mass. Generally, soil respiration correlated positively with soil carbon content. Earthworms significantly increased soil respiration and nitrogen leaching and this was most pronounced in farmyard manure treatments. Microbial activity was generally higher in farmyard manure soil indicating that farmyard manure increases labile organic matter pools in soil. Also, biomass of earthworms and microorganisms was increased in farmyard manure soil. The presence of earthworms reduced microbial biomass, suggesting that earthworms feed on microorganisms or/and that earthworms and soil microorganisms competed for similar organic matter pools in soil. The results demonstrate that NPK fertilisation only is insufficient to sustain O. tyrtaeum, whereas long-term fertilisation with farmyard manure enables survival of endogeic species due to an increased pool of utilisable soil organic matter in arable soil.     

Microbial biomass and respiratory activity in soil aggregates of different sizes from three beechwood sites on a basalt hill

We studied the effects of aggregates of different sizes on the soil microbial biomass. The distribution of aggregate size classes (<2, 2–4, 4–10, >10 mm) in the upper mineral soil horizon (A_h layer) was very different in three sites (upper, intermediate, lower) in a beechwood (Fagus sylvatica) on a basalt hill (Germany). Aggregates of different sizes (<2, 2–4, 4–10 mm) contained different amounts of C and N but the C:N ratios were similar. C and N contents were generally higher in smaller aggregates. The maximum initial respiratory response by microorganisms in intact aggregates and in aggregates passed through a 1-mm sieve declined with the aggregate size, but the difference was more pronounced in intact aggregates. Disruption of aggregates generally increased this response, particularly in 4- to 10-mm aggregates in the lower site. Basal respiration differed strongly among sites, but was similar in each of the aggregate size classes. Aggregate size did not significantly affect the specific respiration (g O₂ g^–1 microbial C h^–1) nor the microbial: organic C ratio, but these parameters differed among sites. Microbial growth was increased strongly by passing the soil through a 1-mm sieve in each of the aggregate materials. The growth of microorganisms in disrupted aggregates was similar, and the effect of aggregate disruption depended on the growth of microorganisms in intact aggregates.     

Microbial biomass phosphorus in soils of beech (Fagus sylvatica L.) forests

Thirty-eight soils from forest sites in central Germany dominated by beech trees (Fagus sylvatica L.) were sampled to a depth of about 10 cm after careful removal of the overlying organic layers. Microbial biomass P was estimated by the fumigation — extraction method, measuring the increase in NaHCO₃-extractable phosphate. The size of the microbial P pool varied between 17.7 and 174.3 g P g^-1 soil and was on average more than seven times larger than NaHCO₃-extractable phosphate. Microbial P was positively correlated with soil organic C and total P, reflecting the importance of soil organic matter as a P source. The mean microbial P concentration was 13.1% of total P, varying in most soils between 6 and 18. Microbial P and microbial C were significantly correlated with each other and had a mean ratio of 14.3. A wide (5.1–26.3) microbial C: P ratio indicates that there is no simple relatinship between these two parameters. The microbial C: P ratio showed strong and positive correlations with soil pH and cation exchange capacity.     

A comparison of soil and microbial carbon,nitrogen, and phosphorus contents,and macro-aggregate stability of a soil under native forest and after clearance for pastures and plantation forest

Total, extractable, and microbial C, N, and P, soil respiration, and the water stability of soil aggregates in the F-H layer and top 20 cm of soil of a New Zealand yellow-brown earth (Typic Dystrochrept) were compared under long-term indigenous native forest (Nothofagus truncata), exotic forest (Pinus radiata), unfertilized and fertilized grass/clover pastures, and gorse scrub (Ulex europaeus). Microbial biomass C ranged from 1100 kg ha^-1 (exotic forest) to 1310kg ha^-1 (gorse scrub), and comprised 1–2% of the organic C. Microbial N and P comprised 138–282 and 69–119 kg ha^-1 respectively, with the highest values found under pasture. Microbial N and P comprised 1.8–7.0 and 4.9–18% of total N and P in the topsoils, and 1.8–4.4 and 23–32%, respectively, in the F-H material. Organic C and N were higher under gorse scrub than other vegetation. Total and extractable P were highest under fertilized pasture. Annual fluxes through the soil microbial biomass were estimated to be 36–85 kg N ha^-1 and 18–36 kg P ha^-1, sufficiently large to make a substantial contribution to plant requirements. Differences in macro-aggregate stability were generally small. The current status of this soil several years after the establishment of exotic forestry, pastoral farming, or subsequent reversion to scrubland is that, compared to levels under native forest, there has been no decline in soil and microbial C, N, and P contents or macro-aggregate stability.